1. Field of the Invention
The invention is related to the field of well log data processing. More specifically, the invention is related to methods for efficiently determining multiple sets of earth formation parameters (models) each of which can cause the same well logging instrument response as is observed on an actual set of well logs.
2. Description of the Related Art
Well log data processing known in the art has as one objective to determine values of physical properties, such as electrical resistivity, of earth formations both in a zone not subject to invasion of wellbore drilling fluids as well as the values of the same physical properties in a so-called "invaded" zone subject to such infiltration of wellbore drilling fluids.
One type of well log data processing is known as "inversion" processing. One form of inversion processing is described, for example in L. R. Lines and S. Treitel, Tutorial: A Review of Least-Squares Inversion and Its Application to Geophysical Problems, Geophysical Prospecting, Vol. 32, pp 159-186 (1984). Inversion processing, for determining electrical resistivity values for example, includes generating an initial estimate, or model, of the resistivity parameters of the earth formations surrounding the well logging instrument in a wellbore. The parameters usually include the vertical thicknesses of the earth formations, the electrical conductivities (resistivities) of the earth formations, and the position of any radial boundaries and the conductivity on either side of such boundary of any particular earth formation having an invaded zone. "Vertical" is typically defined as being coaxial with the well logging instrument. The initial model is used to calculate simulated responses for the individual sensors in the well logging instrument. The simulated instrument responses are then compared with the actual sensor measurements made by the well logging instrument over the same axial interval of the wellbore. Differences between the simulated responses and the measured responses are used to adjust the parameters of the model. Adjustment can include changing the values of resistivity, and positions of axial and radial boundaries in the model. The simulated response calculation and comparison steps are repeated until the differences between the simulated responses and the measured responses fall below a predetermined error threshold. A model which generates simulated responses below the threshold difference, or at a minimum difference, is determined to include the most likely set of earth formation parameters.
A drawback to the inversion processing techniques known in the art is that the solutions they generate may not be unique. This means more than one model of the spatial distribution of the various physical properties of the earth formations could result in well logging instrument responses which are substantially the same as the responses observed in the particular earth formations. It is possible, of course, to test a number of different initial earth models to determine which ones of the different earth models could in fact result in the best match between simulated and observed well logging instrument response. The plurality of earth models can include differences between them such as the number of layers in the earth formations. Testing a plurality of initial earth models can be difficult and time consuming, however, because each initial earth model must undergo a complete inversion process to determine the adjusted or "final" earth model which would result in the observed well logging instrument response.